The present invention relates to a soft decision method and a receiving apparatus in multilevel (amplitude and/or phase) modulation.
In recent years, extensive studies have been made for digital television broadcasting. Among digital signal transmission techniques for digital television broadcasting, an FEC (Forward Error Correction) technique that can improve characteristics by performing soft-decision decoding of received symbols such as maximum-likelihood codes is receiving a lot of attention.
As an example of this technique, a transmission technique using convolutional coding-Viterbi decoding will be examined below. At the transmitter side, as shown in FIG. 1, a data sequence is convolutionally coded (11) and is then mapped and modulated (12) to obtain a transmission signal. On the receiver side, as shown in FIG. 2, the received signal is demodulated and demapped (13), and is then Viterbi-decoded (141) as FEC decoding (14) to obtain decoded data.
In a transmission technique using QPSK as a modulation method, as shown in, e.g., FIG. 3, 8-valued soft decision is done to improve the characteristics as compared to hard decision. In the example, shown in FIG. 3, when the bit is most reliable 0, a soft decision value 0 is set; when the bit is most reliable 1, a soft decision value 7 is set.
However, digital television broadcasting or the like tends to adopt modulation methods using a larger number of codes, but a soft decision method for multilevel (amplitude and/or phase) modulation has not prevailed. Hard decision cannot sufficiently exploit the correction performance of maximum-likelihood codes.